A smaller laser slab can be made to perform comparably to a larger one.

A four-pass optical coupler affords increased (in comparison with related prior two-pass optical couplers) utilization of light generated by a laser diode in side pumping of a solid-state laser slab. The original application for which this coupler was conceived involves a neodymium- doped yttrium aluminum garnet (Nd:YAG) crystal slab, which, when pumped by a row of laser diodes at a wavelength of 809 nm, lases at a wavelength of 1,064 nm.

The Four-Pass Coupler is interposed between the laser diodes and the laser slab. This view is along the longitudinal laser axis. The laser diodes, of which only one is shown here, are arranged in a row parallel to the axis.
Heretofore, typically, a thin laser slab has been pumped in two passes, the second pass occurring by virtue of reflection of pump light from a highly reflective thin film on the side opposite the side through which the pump light enters. In two-pass pumping, a Nd:YAG slab having a thickness of 2 mm (which is typical) absorbs about 84 percent of the 809-nm pump light power, leaving about 16 percent of the pump light power to travel back toward the laser diodes. This unused power can cause localized heating of the laser diodes, thereby reducing their lifetimes. Moreover, if the slab is thinner than 2 mm, then even more unused power travels back toward the laser diodes.

The four-pass optical coupler captures most of this unused pump light and sends it back to the laser slab for two more passes. As a result, the slab absorbs more pump light, as though it were twice as thick. The gain and laser cavity beam quality of a smaller laser slab in conjunction with this optical coupler can thus be made comparable to those of a larger two-pass-pumped laser slab.

The four-pass coupler (see figure) consists of a right-angle polarization cube (RAPC) with a quarter-wave plate on the side facing the laser slab and highly reflective film coating one of the perpendicular sides. The RAPC transmits p-polarized light (light polarized parallel to the plane of incidence) and reflects s-polarized light (light polarized perpendicular to the plane of incidence. Each laser diode emits a collimated beam and is oriented so that the beam is p-polarized (vertically polarized in the figure). The p-polarized beam passes through the RAPC, and then through the quarter-wave plate, which converts it to a rotationally polarized beam. The beam then passes into the laser slab for a first pump pass, reflection, and second pump pass in the usual manner.

The pump light remaining after the second pass leaves the laser slab and travels back into the RAPC via the quarter-wave plate, which converts this light to s polarization. This s-polarized beam is reflected from the internal 45° polarization beam-splitting surface of the RAPC, sending the beam to the reflective coated RAPC surface at normal incidence. After reflection from this surface, this beam is reflected by the 45° surface toward the laser slab and is converted to rotational polarization by the quarter-wave plate. The beam then makes two more passes through the laser slab in the usual manner.

Any pump beam power remaining after the fourth pass is converted to p polarization by the quarter-wave plate and travels back to the laser diode. However, when the coupler is designed correctly in conjunction with the other laser components, the fraction of pump power returning to the laser diode is too small to exert a significant adverse effect on the laser-diode lifetime or performance.

This work was done by Donald B. Coyle of Goddard Space Flight Center.

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